Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus includes an illumination system configured to provide a beam of radiation of radiation; a support configured to support a patterning device configured to impart a pattern to the beam of radiation; a substrate table configured to hold a substrate; a projection system configured to project the patterned beam onto a target portion of the substrate, the projection system and/or illumination system including a focusing element; a plurality of stop discs each having an aperture therethrough different from the size and/or shape of the apertures of the other stop discs; and a mechanism configured to exchangeably place a selected one of the stop discs adjacent to the focusing element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus and to adevice manufacturing method.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning device, which is alternatively referredto as a mask or a reticle, may be used to generate a circuit patterncorresponding to an individual layer of the IC, and this pattern can beimaged onto a target portion (e.g. including part of one or severaldies) on a substrate (e.g. a silicon wafer) that has a layer ofradiation-sensitive material (resist). In general, a single substratewill contain a network of adjacent target portions that are successivelyexposed. Known lithographic apparatus include steppers, in which eachtarget portion is irradiated by exposing an entire pattern onto thetarget portion at once, and scanners, in which each target portion isirradiated by scanning the pattern through the beam of radiation in agiven direction (the “scanning”-direction) while synchronously scanningthe substrate parallel or anti-parallel to this direction.

Between the reticle and the substrate is disposed a projection systemthat images the irradiated portion of the reticle onto the targetportion of the substrate. The projection system includes components todirect, shape and/or control the beam of radiation, and these componentstypically include refractive optics, reflective optics, and/orcatadioptric systems, for example.

A consideration in lithography is the size of features of the patternapplied to the substrate. It is desirable to produce apparatus capableof resolving features as small and close together as possible. A numberof parameters affect the available resolution of features, and one ofthese is the wavelength of the radiation used to expose the pattern.

It is expected that the use of EUV lithography will enable themanufacture of feature sizes down to 32 nm using radiation with an EUV(Extreme Ultra Violet) wavelength between 5 and 20 nm, typically 13.5nm. Radiation at this wavelength is typically strongly absorbed by mostmaterials and conventional refractive optics are generally considered tobe unsuitable for use with such radiation. The optics in a projectionsystem for use with EUV lithography should therefore be based onmirrors, which can only operate in a high vacuum (UHV) environment. Theprojection system is therefore enclosed in a projection optics box (POB)which is kept under vacuum.

Similar considerations apply for lithography using radiation having awavelength falling outside the EUV band. For example, a projectionsystem for lithography using radiation having a wavelength of 193 nm mayalso include mirrors instead of, or in addition to, refractive optics.The POB may therefore need to be kept under a vacuum or at least in acontrolled environment for non-EUV lithography.

Furthermore, considerations that apply to the projection system willalso apply to the illumination system used to supply the beam ofradiation to the reticle. As with the projection system, theillumination system includes components to direct, shape and/or controlthe beam of radiation, and these components typically include refractiveoptics, reflective optics, and/or catadioptric systems, for example. Aswith the projection system, the illumination system may need to be keptin a controlled environment or under vacuum.

The projection system and/or illumination system generally also includeselements to set the numerical aperture, commonly referred to as the“NA”) of the projection system (and/or illumination system. In someprior art systems, an aperture adjustable NA-diaphragm or iris diaphragmmay be provided in a pupil of the projection system and/or illuminationsystem. Particularly in the case of EUV lithography, for most commonprojection optics designs the space around the optical components isvery restricted, making the use of an adjustable diaphragm impractical.Furthermore, it is difficult to provide an elliptical diaphragm foroff-axis reflective systems.

Yet further, an adjustable diaphragm provides substantially circularapertures. However, different shaped apertures can improve imagingperformance for specific structures. For example, an elliptical apertureis useful for off-axis reflective systems. It is not possible to produceapertures having such shape using a conventional diaphragm.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide a solution to theproblem of providing a lithographic apparatus having a variable NA, forexample combining the benefits of a traditional iris diaphragm withadditional, alternative apertures and/or filters.

It is a yet further aspect of the present invention to provide a mode ofimproving imaging performance by pupil shaping/filtering in an EUVprojection system of a lithographic apparatus.

In accordance with one embodiment of the present invention there isprovided a lithographic apparatus including an illumination systemconfigured to provide a beam of radiation of radiation; a supportconfigured to support a patterning device, the patterning deviceconfigured to impart a pattern to the beam of radiation; a substratetable configured to hold a substrate; a projection system configured toproject the patterned beam onto a target portion of the substrate; and astop changer configured to exchangeably place a selected one of aplurality of stop discs adjacent to a focusing element.

The focusing element may be provided in the illumination system or theprojection system.

The focusing element may include a single optical element with afocusing function, e.g. a single lens element or a single (e.g.non-flat) mirror, and may reflect or refract the beam of radiation.

The selected stop disc is exchangeably placeable in or adjacent to apupil plane of the focusing element.

The stop disc is positioned in or near the pupil plane of the projectionsystem or illumination system.

A size of the aperture of the selected stop disc determines a numericalaperture (NA) of the projection system.

Each stop disc may be alternatively termed a “pupil” or “blade”.

The focusing element may reflect or refract the beam of radiation.

The stop changer may include a disc positioning mechanism to selectivelyplace one of the stop discs adjacent the focusing element.

The stop changer may include a disc changing mechanism to select one ofthe stop discs and supplying the selected stop disc to the discpositioning mechanism.

The disc changing mechanism may include a receptacle to store the stopdiscs not supplied to the disc positioning mechanism.

The lithographic apparatus may also include the plurality of stop discs.

The stop discs may each include a differently apertured member, i.e. mayeach include a member having a distinct aperture different from theapertures of the other stop disc members, e.g. in size and/or shape.

Each stop disc may be a first disc having a circular aperture of a firstsize, a second disc having a circular aperture of a second size greaterthan the first size, a disc having an elliptical aperture, a disc havingan aperture with a central obscuration, a disc having a square aperture,a disc having a hexagonal aperture, a disc having a plurality ofapertures, or a disc for correcting pupil apodization. An ellipticallyshaped aperture may correct for Horizontal and Vertical (H-V)orientation differences caused by errors in the illuminator(ellipticity), the projection system or the mask. An aperture with acentral obscuration may increase contrast for many structures bypartially blocking zeroth order light.

An aperture of one or more of the stop discs may be provided with afilter or membrane, which may act as a phase and/or amplitude filter andmay allow for aberration correction. Alternatively an aperture of one ormore of the stop discs may be provided with a mesh.

The selected stop disc may be provided before or in front of thefocusing element such that, in use, radiation passes through theaperture of the selected stop disc prior to reaching the focusingelement. In use, the radiation does not need to thereafter pass backthrough the aperture.

A disc delivery mechanism may be provided to deliver a selected stopdisc from the disc changing mechanism to the disc positioning mechanism.

A magazine may be provided to hold the plurality of stop discs when notin use.

The focusing element may be a reflective lens (i.e. a mirror). Thearrangement described above is particularly appropriate when theradiation in the beam of radiation has a wavelength in the EUV region,i.e. between about 5 and about 20 nm, although it will be appreciatedthat the present invention may be used in conjunction with radiation ofany wavelength, such as in the UV region, e.g. in the range of 5 nm to400 nm and possibly of around 193 nm or 157 nm.

In accordance with a further embodiment of the present invention thereis provided a device manufacturing method including providing a beam ofradiation using an illumination system; patterning the beam ofradiation; projecting the patterned beam of radiation onto a targetportion of a substrate; and adjusting a numerical aperture of a focusingelement of the illumination system or radiation system by exchangeablyplacing a selected stop disc adjacent the focusing element.

In accordance with a still further embodiment of the present invention,a device is manufactured according the method described above.

In accordance with yet another embodiment of the present invention, alithographic apparatus includes an illumination system configured toprovide an EUV beam of radiation; a support configured to support apatterning device, the patterning device configured to impart a patternto the beam of radiation; a substrate table configured to hold asubstrate; a projection system configured to project the patterned EUVbeam of radiation onto a target portion of the substrate; and a stopchanger configured to exchangeably place a selected one of a pluralityof stop discs at a position within the EUV beam of radiation to shapeand/or filter the EUV beam of radiation

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin-film magnetic heads, etc. It shouldbe appreciated that, in the context of such alternative applications,any use of the terms “wafer” or “die” herein may be considered assynonymous with the more general terms “substrate” or “target portion”,respectively. The substrate referred to herein may be processed, beforeor after exposure, in for example a track (a tool that typically appliesa layer of resist to a substrate and develops the exposed resist) or ametrology or inspection tool. Where applicable, the disclosure hereinmay be applied to such and other substrate processing tools. Further,the substrate may be processed more than once, for example, in order tocreate a multi-layer IC, so that the term substrate used herein may alsorefer to a substrate that already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5to 20 nm), as well as particle beams, such as ion beams or electronbeams.

The term “patterning device” used herein should be broadly interpretedas referring to a device that can be used to impart a beam of radiationwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the beam of radiation may not exactly correspond to thedesired pattern in the target portion of the substrate. Generally, thepattern imparted to the beam of radiation will correspond to aparticular functional layer in a device being created in the targetportion, such as an integrated circuit.

Patterning devices may be transmissive or reflective. Examples ofpatterning device include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned.

The support supports, e.g. bears the weight of, the patterning device.It holds the patterning device in a way depending on the orientation ofthe patterning device, the design of the lithographic apparatus, andother conditions, such as for example whether or not the patterningdevice is held in a vacuum environment. The support can be usingmechanical clamping, vacuum, or other clamping techniques, for exampleelectrostatic clamping under vacuum conditions. The support may be aframe or a table, for example, which may be fixed or movable as requiredand which may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device”.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, catadioptric opticalsystems, magnetic, electromagnetic and electrostatic optical systems asappropriate for example for the exposure radiation being used, or forother factors such as the use of an immersion fluid or the use of avacuum. Any use of the term “lens” herein may be considered assynonymous with the more general term “projection system”.

The lithographic apparatus may also encompass various types of opticalcomponents, including refractive, reflective, catadioptric, magnetic,electromagnetic and electrostatic optical components to direct, shape,or control the beam of radiation, and such components may also bereferred to herein, collectively or singularly as a “lens.”

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such multiplestage machines the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index, e.g.water, so as to fill a space between the final element of the projectionsystem and the substrate. Immersion liquids may also be applied to otherspaces in the lithographic apparatus, for example, between the mask andthe first element of the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings, inwhich corresponding reference symbols indicate corresponding parts,wherein

FIG. 1 depicts a lithographic apparatus according to the presentinvention;

FIG. 2 depicts a lithographic apparatus for use with extremeultra-violet (EUV) radiation according to an embodiment of the presentinvention;

FIGS. 3A and 3B depict a mechanism to exchangeably position a selectedstop disc in front of a lens of a projection system of the lithographicapparatus of FIG. 2;

FIGS. 4A, 4B and 4C depict three stages in operation of the mechanism ofFIG. 3A;

FIG. 5 depict the projection system of the lithographic apparatus ofFIG. 2, the projection system including a disc changing mechanism;

FIG. 6 depicts a modified projection system of the lithographicapparatus of FIG. 2, the projection system including a modified discchanging mechanism;

FIG. 7 depicts a further modified projection system of the lithographicapparatus of FIG. 2, the projection system including a further modifieddisc changing mechanism;

FIG. 8 depicts a vacuum lock for enabling a disc to pass into aprojection optics system; and

FIGS. 9(a) to (h) depict front views of a plurality of stop discs orblades for use in the lithographic apparatus of FIG. 2.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a typical lithographic apparatus accordingto the present invention. The apparatus includes an illumination system(illuminator) IL configured to provide a beam of radiation PB (e.g. UVor EUV radiation); a first support (e.g. a mask table) MT configured tosupport a patterning device (e.g. a mask) MA and connected to a firstpositioning device PM that accurately positions the patterning device MAwith respect to a projection system PL; a substrate table (e.g. a wafertable) WT configured to hold a substrate (e.g. a resist-coated wafer) Wand connected to second positioning device PW that accurately positionsthe substrate W with respect to the projection system PL; and theprojection system (e.g. a reflective projection lens) PL configured toimage a pattern imparted to the beam of radiation PB by patterningdevice MA onto a target portion C (e.g. including one or more dies) ofthe substrate W. The patterning device MA is configured to impart thebeam of radiation PB with a pattern in its cross-section.

As here depicted, the apparatus is of a reflective type (e.g. employinga reflective mask or a programmable mirror array of a type as referredto above). Alternatively, the apparatus may be of a transmissive type(e.g. employing a transmissive mask).

The illuminator IL receives radiation from a source SO. The source SOand the lithographic apparatus may be separate entities, for example,when the source SO is a plasma discharge source. In such cases, thesource SO is not considered to form part of the lithographic apparatusand the radiation is generally passed from the source SO to theilluminator IL with the aid of a radiation collector including, forexample, suitable collecting mirrors and/or a spectral purity filter. Inother cases the source SO may be an integral part of the apparatus, forexample, when the source SO is also contained in a vacuum. The source SOand the illuminator IL may be referred to as a radiation system.

The illuminator IL may include adjusting means for adjusting the angularintensity distribution of the beam. Generally, at least the outer and/orinner radial extent (commonly referred to as σ-outer and σ-inner,respectively) of the intensity distribution in a pupil plane of theilluminator IL can be adjusted. The illuminator IL provides aconditioned beam of radiation PB having a desired uniformity andintensity distribution in its cross-section.

The beam of radiation PB is incident on the mask MA, which is held onthe mask table MT. Being reflected by the mask MA, the beam of radiationPB passes through the projection system PL, which focuses the beam ontoa target portion C of the substrate W. With the aid of the secondpositioning device PW and position sensor IF2 (e.g. an interferometricdevice), the substrate table WT can be moved accurately, e.g. so as toposition different target portions C in the path of the beam PB.Similarly, the first positioning device PM and position sensor IF1 (e.g.an interferometric device) can be used to accurately position the maskMA with respect to the path of the beam PB, e.g. after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe object tables MT and WT will be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which form part of the positioning devices PM and PW.However, in the case of a stepper, as opposed to a scanner, the masktable MT may be connected to a short stroke actuator only, or may befixed. Mask MA and substrate W may be aligned using mask alignment marksM1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in the following preferred modes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to the beam ofradiation is projected onto a target portion C at once (i.e. a singlestatic exposure). The substrate table WT is then shifted in the X and/orY direction so that a different target portion C can be exposed. In stepmode, the maximum size of the exposure field limits the size of thetarget portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the beam of radiationis projected onto a target portion C (i.e. a single dynamic exposure).The velocity and direction of the substrate table WT relative to themask table MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width in the non-scanningdirection of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height in the scanningdirection of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the beam of radiation isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 shows a side view of an EUV lithographic apparatus 1 inaccordance with an embodiment of the present invention. It will be notedthat, although the arrangement is different to that of the apparatusshown in FIG. 1, the principle of operation is similar. The apparatus 1includes an illumination system IL having a source-collector module orradiation unit 3, illumination optics unit 4, and a projection systemPL. Radiation unit 3 is provided with a radiation source LA which may beformed by a discharge plasma. The radiation from the radiation unit 3creates a virtual source at an intermediate focus point 12. Theillumination system IL is configured such that the intermediate focus 12is disposed at an aperture 15 in the illumination optics unit 4. Thebeam of radiation 16 is reflected in illumination optics unit 4 viareflective element 13, through an intermediate focus 22, and via afurther reflective element 14 onto a reticle or mask positioned onreticle or mask table MT. A patterned beam 17 is formed which is imagedby projection system PL via reflective elements 18, 19, through anintermediate focus 23 and via reflective elements 24, 25 onto waferstage or substrate table WT. More elements than shown may generally bepresent in the radiation unit 3, illumination optics unit 4 andprojection system PL.

One of the reflective elements 19 acts as a pupil and therefore has infront of it an NA stop disc 20 having an aperture 21 therethrough. Thesize of the aperture 21 determines the angle α_(i) subtended by the beam17 of radiation as it strikes the substrate table WT. This angle givesrise to a parameter involved in the projection process, the NumericalAperture (NA), which is defined asNA=n sin α_(i)where n is the refractive index of the medium surrounding the substratetable WT. Using EUV the projection system is operated under vacuum, inwhich n=1.000, and even in air n=1.003, so in general the NumericalAperture can be defined by NA≈sin α_(i).

The value of NA controls the resolution of the apparatus. The resolutioncan be represented as the critical dimension (CD) of the smallestfeature which can be imaged, and this is determined by${CD} = {k_{1}\frac{\lambda}{NA}}$where k₁ is a constant. Thus it can be seen that the higher the value ofNA, the better the resolution, i.e. the smaller the feature which can beimaged.

However, as NA is increased the depth of focus (DOF) of the projectionsystem PL is decreased. Therefore for operations which do not requireexceptionally high resolution, it is desirable to reduce the value of NAso as to increase the depth of focus and thus the tolerance of thepositioning of the substrate on the substrate table WT.

The aperture is used to capture radiation diffracted from the reticleMA. This diffracted radiation is used for image formation. The extent ofthe diffraction depends on the size of the features on the reticle. Thusit can be seen that the user needs to be able to decrease or increasethe NA. This is achieved by replacing the NA stop disc 20 with anotherstop disc having a different aperture therethrough. The differentaperture changes the angle α_(i) subtended by the radiation beam 17 atthe substrate table WT.

The NA stop disc 20 is mounted on a disc positioning mechanism 32 shownin plan view in FIG. 3A and in elevation in FIG. 3B. The discpositioning mechanism 32 includes a lift in the form of an elongate arm34 ending in a fork 35. The stop disc 20 is supported on the fork 35.The other end of the elongate arm 34 is mounted on a support 36 via apivot 37 which allows the arm 34 to pivot in the vertical plane. Thesupport 36 is mounted on a base frame of the projection system PL.

A second arm 38 is rigidly attached to the support 36. The second arm 38ends in a second fork 39, against which the stop disc 20 abuts when itis in the operational position. FIG. 3B shows the fork 35 in theoperational position 31 using solid lines and in the loading position 33using dashed lines. In operation, the second fork 39 is placed adjacentto the reflective element 19 whose NA is to be controlled.

As shown in FIG. 3A, there is associated with the disc positioningmechanism 32 a delivery mechanism 41 for introducing the stop disc 20 tothe positioning mechanism 32. The delivery mechanism includes a shovelportion 42 upon which the stop disc 20 rests, mounted on a handling arm43 which moves the shovel 42 in a horizontal direction. The shovelportion 42 includes a fork 44 whose arms are sufficiently widely spacedto allow fork 35 of the positioning mechanism to pass up between them,thus lifting the stop disc 20 off the shovel portion 42 and up into theoperational position abutting the second fork 39.

This operation is shown in more detail in FIGS. 4A, 4B and 4C. In thearrangement shown in FIG. 4A the stop disc 20 rests on the shovelportion 42 of the delivery mechanism 41. The shovel portion 42 is movedtowards the positioning mechanism 32 by the handling arm 43. The arm 34and fork 35 of the positioning mechanism 32 are in the loading position33 shown in FIG. 3B.

FIG. 4B shows the situation when the shovel portion 42 and stop disc 20reach the delivery mechanism 32. The shovel portion 42 and stop disc 20move to a position directly above the fork 35. The elongate arm 34pivots about the pivot point 37 so as to move the fork 35 up through thefork 44 of the shovel portion 42, thus lifting the stop disc 20 off theshovel 42. This moves the stop disc 20 into the operational position 31shown in FIG. 3B.

FIG. 4C shows the situation when the stop disc 20 is in the operationalposition 31. Once the stop disc 20 has been lifted clear of the shovelportion 42 by the fork 35, the delivery mechanism 41 is retracted awayfrom the positioning mechanism 32.

When a stop disc 20 is to be replaced, the above procedure takes placein reverse. The shovel portion 42 of the delivery mechanism 41 is movedinto position beneath the stop disc 20 and fork 35 of the positioningmechanism. The stop disc 20 is lowered by the fork 35 as the elongatearm 34 pivots, and comes to rest on the fork 44 of the shovel portion42. The fork 35 of the positioning mechanism 32 moves down through thefork 44 of the shovel 42 into the loading position 33. The shovelportion 42 and stop disc 20 are then withdrawn using the horizontal arm43. Another stop disc may then be placed on the shovel portion 42 andinserted into position as described above.

Alternatives to the positioning mechanism 32 and delivery mechanism 41described above can be envisaged. For example, the positioning mechanism32 could include a fixed table (similar to the fork 35) on which thestop disc 20 is to be placed. The delivery mechanism 41 could be movablein a vertical as well as horizontal direction, so that it can move thestop disc horizontally into place and then lower it onto the fixed tablebefore being retracted. Any suitable delivery mechanism and positioningmechanism may be used.

FIG. 5 is a schematic depiction of the projection system PL locatedinside a controlled or purged environment or vacuum chamber 46. An NAstop disc 20 mounted on a disc positioning mechanism 32 (as describedabove) is shown in schematic form. The reflective element in front ofwhich the stop disc 20 is mounted is not shown. Associated with the discpositioning mechanism 32 is a disc changing mechanism 47 and a magazine48 supporting a plurality of stop discs.

When it is desired to change the NA stop of the reflective element, thestop disc 20 is removed from the operational position by the positioningmechanism 32. The stop disc 20 is then withdrawn by a delivery mechanism41 to the disc changing mechanism 47 and placed in the magazine 48.Another stop disc 20 is then selected from the magazine 48 andtransported via the delivery mechanism 41 to the positioning mechanism32. The other stop disc 20 is then put in place in front of thereflective element.

It will be noted that in the arrangement shown in FIG. 5, the discchanging mechanism 47 and magazine 48 are located within the projectionsystem PL which is located within vacuum chamber 46.

FIG. 6 is a schematic depiction of a modified projection system PL anddisc changing system 49, both mounted inside a vacuum chamber 46. Thedisc changing system 49 is mounted external to the projection system PL,either on the vacuum chamber 46 or on a cooling system (not shown) forthe projection system PL. The disc changing system 49 includes a discchanging mechanism 47 and magazine 48 like those shown in FIG. 5,together with a delivery mechanism 41 which transfers a stop disc 20from the disc changing mechanism 47 outside the projection system PL toa disc positioning system 32 inside the projection system PL. Placingthe disc changing system 49 outside the projection optics system 45allows much easier access for repair and to change the stop discs heldin the magazine 48.

FIG. 7 is a schematic depiction of a further modified projection systemPL and disc changing system 49. The arrangement is similar to that shownin FIG. 6 except that the disc changing system 49 is located outside thevacuum chamber 46 enclosing the projection system PL. The deliverymechanism 41 passes through a vacuum valve 50 to enable a stop disc 20to be passed from the disc changing system 49 into the vacuum chamber 46without breaking the vacuum. This arrangement allows access to the discchanging mechanism 47 and magazine 48 without the need to break thevacuum in the projection optics system. This allows for much easierrepair of the mechanism or replacement of NA stop discs.

FIG. 8 is a schematic diagram of the vacuum valve 50 which enables thestop disc to be passed from the disc changing system 49 into the vacuumchamber 46 without breaking the vacuum. The vacuum valve 50 includes anairlock chamber 53 having a valve 51, 52 at each end. Initially the stopdisc 20, resting on the shovel portion 42 of the delivery mechanism 14,is contained within the disc changing system 49. The first valve 51 isopened and the disc is inserted into the airlock chamber. The firstvalve 51 is then closed around the handling arm 43 of the deliverymechanism 41 and the airlock chamber is evacuated until it is undervacuum. The second valve 52 is then opened and the delivery mechanism 41passes the stop disc into the vacuum chamber 46 and presents it to thepositioning mechanism 32 as described earlier. The shovel portion isthen retracted into the airlock chamber 53 and the second valve 52closed. The airlock chamber is then brought up to atmospheric pressure,the first valve 51 opened and the shovel retracted into the discchanging mechanism 49. To remove a stop disc from the vacuum chamber 46the above sequence of events is operated in reverse.

Optionally the disc changing system 49 may be enclosed within a secondvacuum chamber (not shown), separate from the vacuum chamber 46enclosing the projection system PL. This arrangement allows the discchanging mechanism 47 and magazine 48 to be kept under vacuum, reducingthe risk of contamination.

Referring now to FIGS. 9(a) to (h), there are shown a plurality of stopdiscs 20 a to 20 h, which can be provided in the projection systems PLof FIGS. 5, 6 and/or 7. Each disc 20 a to 20 h includes a member havinga different aperture. Stop disc 20 a has a circular aperture of a firstsize, stop disc 20 b has a circular aperture of a second size greaterthan the first size, stop disc 20 c has an elliptical aperture, stopdisc 20 d has an aperture with a central obscuration.

Stop disc 20 e has a square aperture, stop disc 20 f has a hexagonalaperture, and stop disc 20 g has a plurality of circular apertures.

An elliptical aperture is especially useful for off-axis reflective (orcatadioptric) systems (e.g. EUV projection systems) since they are, bynature, slightly elliptical. This means that errors made within theprojection system PL can be compensated for within the module.Refractive projection systems are on-axis and therefore in general notelliptical. An elliptical aperture, however, could be used to correcterrors from other modules, e.g. ellipticity of the illuminator,Horizontal-Vertical (H-V) bias on the mask (unwanted difference oflinewidth with orientation).

It will be appreciated that the aperture shapes of the stop discs 20 ato 20 h of FIG. 9, are just examples of likely or possible shapes. Inprinciple, all kinds of shapes of aperture (and obscuration area) couldbe provided. For example, square or hexagonal apertures. An opticalshape for any given situation will be a function of illumination modeand structure to be imaged. It is furthermore possible that the shape ofthe aperture of the selected stop disc can be optimized together withthe illumination mode and/or mask layout.

Another effect which can be corrected by the present invention is lensapodization. This is the effect that the lens transmission depends onthe position in the lens pupil through which the light ray passes.Correction is possible with a filter in the pupil having a certaintransmission profile. One implementation would be a fine mesh withvarying mesh density leading to the desired transmission correction.

The aperture of any of the stop discs 20 a to 20 h can be provided witha filter member or membrane 21 a. This is a piece of correction opticsthat corrects aberrations in the pupil plane. For a transmissive systema plane-plate can be used with extremely small thickness variations (ofthe order of nm's). Most effective is the correction of higher orderaberrations for which no lens manipulators exist. Aberrations induced bythe thickness of the plate, should be corrected for during lens set-upor, possibly, taken into account during lens adjustment and/or design.For EUV the membrane has to be extremely thin (<100 nm for Si) to limitthe absorption. A net-like support frame may be necessary.

Stop disc 20 h has a circular aperture and may be used to correct forpupil apodization. In the aperture a wire mesh (or something similar) isprovided. By creating a mesh with a variable transmission, thetransmission over the pupil can be tuned. In FIG. 9(h) there are twoarea's with a different transmission. In most cases, however, a gradualvariation over the pupil should be provided. A mesh with variabletransmission can be created by varying the width of the mesh wires orthe density of the mesh. Additional requirements to the mesh may includethat: the nominal transmission should be close to 100% (implying thatthe mesh is not dense) and it should not significantly influence theangular distribution of the radiation passing it.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

Particularly, it will be appreciated that the present invention proposesa solution to the problem of unsuitability of adjustable diaphragms byproviding a series of individual stop discs, each having an opening witha fixed aperture, to adjust the NA stop. The stop discs are loaded intoa mechanical disc changer mechanism (analogous to a “juke box”), whichcan place one of the discs at a time in the pupil of the projectionsystem. A single stop disc occupies considerably less space than anadjustable diaphragm, and enables the shape of the NA stop to bedetermined more accurately.

It will be appreciated that departures from the above describedembodiments may still fall within the scope of the invention. Forexample, the lithographic apparatus described includes a reflectivereticle and a projection system including reflective elements, but atransmissive reticle and/or elements in the projection system may alsobe used. Furthermore, the apparatus has been described for use with EUVradiation but it will be appreciated that radiation of otherwavelengths, for example 193 nm, may also be used.

In addition, the description above describes the arrangements requiredto change the NA of the projection system of a lithographic apparatus.The illumination system also has a numerical aperture and the methodsand apparatus used to control the NA of the projection system canequally be used to control the NA of the illumination system.

In a further modification, it will be understood that the projectionsystem may be operated in a controlled environment which does notinclude a vacuum. For example, some projection systems operate in asystem purged with nitrogen gas. The systems described above for movingstop discs into and out of a vacuum chamber apply equally to anycontrolled environment. It may not even be necessary to provide anenclosed chamber. For example, in purge systems in which the gas isheavier or lighter than air, one or more walls can be omitted.

1. A lithographic apparatus, comprising: an illumination systemconfigured to provide a beam of radiation; a support configured tosupport a patterning device, the patterning device configured to imparta pattern to the beam of radiation; a substrate table configured to holda substrate; a projection system configured to project the patternedbeam onto a target portion of the substrate; and a stop changerconfigured to exchangeably place a selected one of a plurality of stopdiscs adjacent to a focusing element of the lithographic apparatus.
 2. Alithographic apparatus as claimed in claim 1, wherein the focusingelement is provided in the projection system or the illumination system.3. A lithographic apparatus as claimed in claim 1, wherein the selectedstop disc is exchangeably placeable in or adjacent to a pupil plane ofthe focusing element.
 4. A lithographic apparatus as claimed in claim 1,wherein the selected stop disc is exchangeably placeable in or near apupil plane of the illumination system or the projection system.
 5. Alithographic apparatus as claimed in claim 1, wherein the selected stopdisc is provided in the projection system and a size of an aperture ofthe selected stop disc determines a numerical aperture of the projectionsystem.
 6. A lithographic apparatus as claimed in claim 1, wherein thefocusing element reflects and/or refracts the beam of radiation.
 7. Alithographic apparatus as claimed in claim 1, wherein the stop changercomprises a disc positioning mechanism for selectively placing one ofthe stop discs adjacent the focusing element.
 8. A lithographicapparatus as claimed in claim 7, wherein the stop changer comprises adisc changing mechanism configured to select one of the stop discs andprovide the selected stop disc to the disc positioning mechanism.
 9. Alithographic apparatus as claimed in claim 8, wherein the disc changingmechanism comprises a receptacle for storing the stop discs not suppliedto the disc positioning mechanism.
 10. A lithographic apparatus asclaimed in claim 1, further comprising the plurality of stop discs. 11.A lithographic apparatus as claimed in claim 1, wherein the plurality ofstop discs each comprise an aperture different from the apertures ofeach of the other stop discs.
 12. A lithographic apparatus as claimed inclaim 1, wherein the selected stop disc is a first disc having acircular aperture of a first size, a second disc having a circularaperture of a second size greater than the first size, a disc having anelliptical aperture, a disc having an aperture with a centralobscuration; a disc having a square aperture, a disc having a hexagonalaperture, a disc having a plurality of apertures, or a disc forcorrecting pupil apodization.
 13. A lithographic apparatus as claimed inclaim 1, wherein an aperture of at least one of the plurality of stopdiscs is provided with a filter or membrane or mesh.
 14. A lithographicapparatus as claimed in claim 1, wherein the selected stop disc isprovided before the focusing element such that, in use, radiation passesthrough the aperture of the selected stop disc prior to reaching thefocusing element.
 15. A lithographic apparatus as claimed in claim 8,further comprising a disc delivery mechanism configured to deliver aselected stop disc from the disc changing mechanism to the discpositioning mechanism.
 16. A lithographic apparatus as claimed in claim8, further comprising a magazine configured to hold the plurality ofstop discs when not in use.
 17. A lithographic apparatus as claimed inclaim 1, wherein the focusing element is a lens, a reflective lens, or amirror.
 18. A lithographic apparatus as claimed in claim 1, wherein thebeam of radiation has a wavelength between about 5 nm and about 400 nm.19. A lithographic apparatus as claimed in claim 1, wherein the beam ofradiation has a wavelength between about 5 nm and about 20 nm.
 20. Adevice manufacturing method, comprising: providing a beam of radiationusing an illumination system; patterning the beam of radiation;projecting the patterned beam of radiation onto a target portion of asubstrate at least partially covered by a layer of radiation sensitivematerial using a projection system; and adjusting a numerical apertureof a focusing element of the illumination system or radiation system byexchangeably placing a selected stop disc adjacent the focusing element.21. A method according to claim 20, wherein adjusting the numericalaperture further comprises: removing the selected stop disc from aposition adjacent the focusing element; and exchanging the selected stopdisc with a further selected stop disc having an aperture different thanan aperture of the selected stop disc.
 22. A device manufacturedaccording to the method of claim
 21. 23. A lithographic apparatus,comprising: an illumination system configured to provide an EUV beam ofradiation; a support configured to support a patterning device, thepatterning device configured to impart a pattern to the beam ofradiation; a substrate table configured to hold a substrate; aprojection system configured to project the patterned EUV beam ofradiation onto a target portion of the substrate; and a stop changerconfigured to exchangeably place a selected one of a plurality of stopdiscs at a position within the EUV beam of radiation to shape and/orfilter the EUV beam of radiation.